AVULSION NODES LOCATED BY INFLUXES OF SEDIMENT WITH HIGHER RESISTANCE: INDICATIONS FROM THE HIGH-SHEAR STRESS FLUME AND HISTORICAL DATA

Prior to the construction of Elephant Butte Dam, the Rio Grande avulsed frequently, approximately once per decade during the 1850’s through the 1890’s. The frequent avulsions may be related to the influx of coarser-grained sediment from large arroyos entering the Rio Grande Valley. In this instance, avulsion may be more related to channel obstruction than to straightening of a channel or due to build-up of sediment in a meander belt.

A mobile high-shear stress flume was used to model the susceptibility to erosion of sediment at three sites in the channel of the Rio Grande along the U.S. Mexico border between El Paso and Ft. Quitman, Texas. At two arroyos, Alamo and Balluco, the rapid deposition of gravelly sand at the arroyo mouths created bars that were over twice as difficult to erode as the normal channel fill. Upstream of Balluco arroyo the Rio Grande was dammed and formed a pond 1.3 m above its normal elevation. Other effects were downstream widening of the channel and the formation of mid-channel and side-channel bars. This highlights a natural process that probably effects the Rio Grande along much of its length, accretion of more poorly erodable bars within the channel during low-flow years that create dams. During floods, the river may avulse because the floodplain sediments are more easily erodable than the plug at the arroyo mouth.